LAUSR

Abstract The microstructure variation in Al-Si-Mg alloys added with high content of Cu during aging process is investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). At the early stage of aging process, there are needle-like GP zone and β ″ containing Cu precipitating in succession. A kind of bright contrast step type structure unit (step-unit), consisting of Cu atoms, is observed in the edge of these precipitates. The amount of step-unit increases with precipitates thickening. Step-unit is resulted by the translation distribution of a kind of atomic coordination geometry and the specific occupation of Cu atoms in the precipitates. This kind of atomic coordination is called Unit T in β ' ' and Unit T' in GP zone, respectively. The thickening process of these needle-like precipitates is realized by the formation of Unit T ( T ′ ) and local diffusion of solute atoms.